btrfs: fiemap: Cache and merge fiemap extent before submit it to user
[ Upstream commit 4751832da990a927c37526ae67b9226ea01eb99e ]
[BUG]
Cycle mount btrfs can cause fiemap to return different result.
Like:
# mount /dev/vdb5 /mnt/btrfs
# dd if=/dev/zero bs=16K count=4 oflag=dsync of=/mnt/btrfs/file
# xfs_io -c "fiemap -v" /mnt/btrfs/file
/mnt/test/file:
EXT: FILE-OFFSET BLOCK-RANGE TOTAL FLAGS
0: [0..127]: 25088..25215 128 0x1
# umount /mnt/btrfs
# mount /dev/vdb5 /mnt/btrfs
# xfs_io -c "fiemap -v" /mnt/btrfs/file
/mnt/test/file:
EXT: FILE-OFFSET BLOCK-RANGE TOTAL FLAGS
0: [0..31]: 25088..25119 32 0x0
1: [32..63]: 25120..25151 32 0x0
2: [64..95]: 25152..25183 32 0x0
3: [96..127]: 25184..25215 32 0x1
But after above fiemap, we get correct merged result if we call fiemap
again.
# xfs_io -c "fiemap -v" /mnt/btrfs/file
/mnt/test/file:
EXT: FILE-OFFSET BLOCK-RANGE TOTAL FLAGS
0: [0..127]: 25088..25215 128 0x1
[REASON]
Btrfs will try to merge extent map when inserting new extent map.
btrfs_fiemap(start=0 len=(u64)-1)
|- extent_fiemap(start=0 len=(u64)-1)
|- get_extent_skip_holes(start=0 len=64k)
| |- btrfs_get_extent_fiemap(start=0 len=64k)
| |- btrfs_get_extent(start=0 len=64k)
| | Found on-disk (ino, EXTENT_DATA, 0)
| |- add_extent_mapping()
| |- Return (em->start=0, len=16k)
|
|- fiemap_fill_next_extent(logic=0 phys=X len=16k)
|
|- get_extent_skip_holes(start=0 len=64k)
| |- btrfs_get_extent_fiemap(start=0 len=64k)
| |- btrfs_get_extent(start=16k len=48k)
| | Found on-disk (ino, EXTENT_DATA, 16k)
| |- add_extent_mapping()
| | |- try_merge_map()
| | Merge with previous em start=0 len=16k
| | resulting em start=0 len=32k
| |- Return (em->start=0, len=32K) << Merged result
|- Stripe off the unrelated range (0~16K) of return em
|- fiemap_fill_next_extent(logic=16K phys=X+16K len=16K)
^^^ Causing split fiemap extent.
And since in add_extent_mapping(), em is already merged, in next
fiemap() call, we will get merged result.
[FIX]
Here we introduce a new structure, fiemap_cache, which records previous
fiemap extent.
And will always try to merge current fiemap_cache result before calling
fiemap_fill_next_extent().
Only when we failed to merge current fiemap extent with cached one, we
will call fiemap_fill_next_extent() to submit cached one.
So by this method, we can merge all fiemap extents.
It can also be done in fs/ioctl.c, however the problem is if
fieinfo->fi_extents_max == 0, we have no space to cache previous fiemap
extent.
So I choose to merge it in btrfs.
Signed-off-by: Qu Wenruo <quwenruo@cn.fujitsu.com>
Reviewed-by: Liu Bo <bo.li.liu@oracle.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>
Signed-off-by: Sasha Levin <sashal@kernel.org>
1 file changed